The person spoke to the AP on the condition of anonymity Tuesday because the decision has not been announced. ESPN was first to report Horford's decision.
A five-time All-Star, the 33-year-old Horford joins point guard Kyrie Irving, who also won't exercise his option to remain in Boston for the final year of his contract. Horford signed a four-year, $113 million deal in 2016.
Among the preseason favorites in the Eastern Conference before last season, the Celtics finished the regular season 49-33. They were eliminated 4-1 in the conference semifinals by the Milwaukee Bucks.
Horford was one of Boston's steadiest players throughout, averaging 13.9 points and nine rebounds in the postseason.
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